Biomedical Engineering Reference
In-Depth Information
when that chemistry is highly unusual (such as
ion-exchange properties), or perhaps when ordi-
nary chemistry is densely packed on the surface
(such as carboxyl-terminated SAMs). There is no
doubt that surface chemistry and wettability are
inextricably convolved properties, because it is
the hydrogen bonding of water to surface func-
tional groups that most profoundly influences
wettability; see, as examples, Refs.
19-21
. Wet-
ting and surface chemistry are not separate fac-
tors, as is sometimes asserted in the biomaterials
literature
[87]
. Indeed, surface chemistry is
responsible for wetting properties. But it appears
that sometimes surface chemistry does shine
through the generic effect on water wettability.
Returning to
Figure 8.4
, any surface chemis-
try, whether homogeneous or heterogeneous,
interacts with water in a manner particular to
that surface chemistry, and a dynamic interphase
will be formed by contact with a biological
milieu. Unless that interphase happens to prop-
erly direct subsequent interactions with the bio-
logical milieu, a homogeneous surface chemistry
will not necessarily lead to improved biocompat-
ibility. Although SAM chemistries yet may prove
to be a necessary part of an effective surface
engineering strategy, well-defined surfaces are
not likely to be sufficient in and of themselves.
A biomimetic component seems necessary to
carry out the function “control of interactions
with components of living systems.”
for example, would influence the biological
response to a material in a different way than
observed using a macroscopic surface of either
surface-chemical scheme. Likewise, topographi-
cal features (surface textures) have been found
to influence the behavior of biology at textured
surfaces
[92]
. Biomaterials quickly embraced
this approach to engineering biocompatibility
and functionality
[93-95]
.
The length scale of patterns can range from
centimeter to nanometer
[93]
, depending on
application and purpose. For example, surface
chemistry or texture patterns can be used to cre-
ate islands onto which cells specifically adhere in
a sea of non-adherent surface
[96]
. When surface
patterns approach the size scale of cells or bio-
logical molecules, the biological response can be
much more than the sum or average of effects
obtained on macroscopic surfaces consisting of a
single surface chemistry or texture type. As exam-
ples, it is found that microscale chemical patterns
on a surface can control cell shape and pheno-
typic behavior
[90]
. Nano-patterned surface
chemistry is found to greatly affect blood plasma
coagulation induced by contact with these sur-
faces compared to activation of coagulation by
equal surface area of macroscopic uniform chem-
istries comprising the pattern
[97]
. Likewise, it is
found that activation of blood plasma coagula-
tion induced by surfaces with molecularly dis-
persed chemical functionalities is different than a
physical mixture of uniform surface chemistry
with the same net hydrophilicity
[98]
.
8.3.3 Surface Chemical Patterns
and Textures
Not long after it was discovered that surface
chemistry can influence the biological response
to materials, the idea of creating patterns of
chemistry at various size scales on a surface was
pursued (see Refs.
88-90
as examples), leading
to a rapidly inflating literature on the subject for
many different applications
[91]
. The idea here
is that an alternating presentation of a particular
surface chemistry mixed with a second particu-
lar chemistry, such as a checkerboard pattern,
8.3.3.1 The Biological Response to Surface
Chemical Patterns and Textures
The motivation behind deliberately imposed
surface heterogeneity in the form of surface
patterns and textures is almost the opposite of
that discussed in Section
8.3.2
for SAM surfaces.
The motivation for well-defined surfaces was
that chemical heterogeneity was responsible for
poor biocompatibility. The motivation behind
ordered patterns of heterogeneity is that hetero-
geneity will somehow direct favorable surface
